Polymethylmethacrylate is a strong, optically clear, weather-resistant polymer, but as conventionally made by free-radical polymerization, either in the presence or absence of cross-linkers, is quite brittle and deficient in impact strength. For that reason, its use is limited to applications in which only very modest impact strength is required. To increase its range of application, polymethylmethacrylate should have high impact strength and should include qualities of resistance to ultraviolet and visible radiation and oxidative degradation in order to provide weatherability.
The highest quality polymethylmethacrylate sheet with the smoothest surface is made by a casting process, in which the methyl methacrylate monomer is polymerized either batchwise in a cell or continuously between two continuous belts. A readily castable high-impact polymethylmethacrylate composition of good weatherability is needed in the art.
Co-pending application U.S. Ser. No. 661,831 describes the preparation of a grafted acrylic copolymer in emulsion which can be dried, dispersed in a mixture of acrylic monomers, and cast to produce a sheet with enhanced impact strength and good weatherability. The amount of modifier that can be incorporated in this way is severely limited by the maximum practical viscosity of the casting mixture, and for that reason the impact strength attainable is also limited. See also U.S. Pat. No. 3,793,402, which describes preparation of impact-modified polymethylmethacrylate by extrusion blending a graft emulsion polymer with a polymethylmethacrylate/ethyl acrylate molding powder. It also describes casting of the molding powder after sheeting a mixture of emulsion polymer powder with a methyl methacrylate/ethyl acrylate monomer mixture and initiator on a cold roll mill. This casting procedure presents severe difficulties in implementation in casting operations because of the very high viscosity of the monomer/graft polymer mixture.
A second inherent limitation of conventional acrylic modifiers is in maintaining impact strength at low temperatures, because of their relatively high glass transition temperatures.
Other high-impact acrylic resins modified with polybutadiene and/or styrene-butadiene have been described. See U.S. Pat. Nos. 2,857,360, 3,029,222, and 3,261,887. Because of the unsaturated carbon linkages, these are more susceptible to oxidation and weathering than polymer modified with acrylic rubber, as pointed out by Manson et al, "Polymer Blends and Composites", pp. 117-119, Plenum, NY (1976).
Ethylene-propylene or ethylene-butylene rubbers offer possibilities for combining glass transition temperatures lower than those of acrylic rubbers with weatherability. A difficulty with these materials is in making a stable dispersion of appropriate particle size (0.2-5 microns) for maximum impact strength, since they are not readily compatible with polymethylmethacrylate, or soluble in the monomer.
In addition to the above-mentioned U.S. Pat. No. 3,793,402, the reader may be interested in reviewing Yusa et al U.S. Pat. No. 3,922,321, which recites improvements in weatherability as well as impact strength; however, contrary to the solution to the problem offered by the applicant herein, Yusa et al employ a methyl methacrylate-grafted butyl acrylate/styrene, a different material entirely. Also of interest will be various patents to Gergen and Davison, i.e. U.S. Pat. Nos. 4,085,163 (see col. 18, line 58), 4,081,424 (col. 19, line 50), 4,111,894; 4,110,303 and 4,102,854; however, those disclosures which mention methacrylates utilize them only as more or less incidental comonomers in the specifically described nitrile barrier resins. The Lunk U.S. Pat. No. 3,810,957 may also be of interest for its recitation of various impact modifiers for use in thermoplastics.
The use of styrene-(ethylene-butene-1)-styrene block copolymer as an addition for scrap blends of polystyrene and polypropylene is described in the December, 1981 issue of Modern Plastics, pp. 60-62.
Kelsey in U.S. Pat. No. 4,080,406 discloses transparent high-impact compositions obtained by polymerizing a mixture of methyl methacrylate, higher methacrylates and styrene with styrene-butadiene block copolymer and/or polybutadiene. Here the unsaturation in the rubber provides a source of good grafting sites, helpful in stabilizing the rubbery phase during polymerization and subsequent processing but detrimental to weatherability. Harlan, Jr. in U.S. Pat. No. 4,007,311 describes the polymerization of acrylates or methacrylates with styrene-butadiene-styrene block copolymers by polymerizing methylmethacrylate in the presence of peroxides, also analogous reactions when the butadiene portion of the block has been hydrogenated, and their use in adhesives. Falk in U.S. Pat. No. 4,212,958 discloses the preparation of graft/polyblend compositions by polymerizing acrylic esters in the presence of saturated rubbers, including ethylene-propylene rubber, hydrogenated styrene-butadiene blocks, and hydrogenated polybutadiene with benzoyl peroxide in emulsion and solution. The polymers were used as flow and impact modifiers for PVC. Kitagawa et al in U.S. Pat. No. 4,287,317 disclose a continuous process for producing impact-modified polymethylmethacrylate, using saturated or unsaturated rubbers involving the preparation of a prepolymer syrup with dispersed rubber particles, which can subsequently be cast to produce an impact-modified polymethylmethacrylate. Moran in U.S. Pat. No. 4,097,555 discloses a method for producing transparent, high impact compositions containing an alkenyl aromatic-alkenyl nitrile acrylic matrix and styrene-butadiene-styrene block copolymer as a rubbery phase (unsaturated).